1. Field of Invention
Embodiments of the invention relate generally to magnetic random access memory, and more particularly, to Spin Torque Transfer Magnetic Random Access Memory (STT-MRAM).
2. Description of Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light and not as admissions of prior art.
Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. MRAM differs from volatile Random Access Memory (RAM) in several respects. Because MRAM is non-volatile, MRAM can maintain memory content when the memory device is not powered. Though non-volatile RAM is typically slower than volatile RAM, MRAM has read and write response times that are comparable to that of volatile RAM. Unlike typical RAM technologies which store data as electric charge, MRAM data is stored by magnetoresistive elements. Generally, the magnetoresistive elements in an MRAM cell are made from two magnetic regions, each of which holds a magnetization. The magnetization of one region (the “pinned region”) is fixed in its magnetic orientation, and the magnetization of the other region (the “free region”) can be changed by an external magnetic field generated by a programming current. Thus, the magnetic field of the programming current can cause the magnetic orientations of the two magnetic regions to be either parallel, giving a lower electrical resistance across the magnetoresistive elements (“0” state), or antiparallel, giving a higher electrical resistance across the magnetoresistive elements (“1” state) of the MRAM cell. The switching of the magnetic orientation of the free region and the resulting high or low resistance states across the magnetoresistive elements provide for the write and read operations of the typical MRAM cell.
Though MRAM technology offers non-volatility and faster response times, the MRAM cell is limited in scalability and susceptible to write disturbances. The programming current employed to switch between high and low resistance states across the MRAM magnetic regions is typically high. Thus, when multiple cells are arranged in an MRAM array, the programming current directed to one memory cell may induce a field change in the free region of an adjacent cell. This potential for writes disturbances, also known as the “half-select problem,” can be addressed using a spin torque transfer technique.
A conventional spin torque transfer MRAM (STT-MRAM) cell may include a magnetic cell stack, which may be a magnetic tunnel junction (MTJ) or a spin valve structure. An MTJ is a magnetoresistive data storing element including two magnetic regions (one pinned and one free) and nonmagnetic region in between, which may be accessed through data lines (e.g., bit line, word line, source line, etc.) and an access transistor. A spin valve has a structure similar to the MTJ, except a spin valve has a conductive region in between the two magnetic regions.
A programming current typically flows through the access transistor and the magnetic cell stack. The pinned region polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the stack. The spin-polarized electron current interacts with the free region by exerting a torque on the free region. When the torque of the spin-polarized electron current passing through the stack is greater than the critical switching current density (Jc), the torque exerted by the spin-polarized electron current is sufficient to switch the magnetization of the free region. Thus, the magnetization of the free region can be aligned to be either parallel or antiparallel to the pinned region, and the resistance state across the stack is changed.
The STT-MRAM has advantageous characteristics over the MRAM, because the spin-polarized electron current eliminates the need for an external magnetic field to switch the free region in the magnetoresistive elements. Further, scalability is improved as the programming current decreases with decreasing cell sizes, and the write disturbance and half-select problem is addressed. Additionally, STT-MRAM technology allows for a higher tunnel magnetic resistance ratio, meaning there is a larger ratio between high and low resistance states, which may improve the accuracy of read operations in the magnetic domain.
However, high programming current densities through the STT-MRAM cell may still be problematic. High current densities through the magnetic regions of the memory cell may increase the energy consumption in the cell and the thermal profile in the regions, affecting the cell's integrity and reliability, and may also lead to larger silicon real estate consumption for each cell.